The Making of Species 



beast of prey and its victim are evenly matched 

 as regards fleetness and power of endurance that 

 small variations in these qualities can have a 

 survival value. But in the rough and tumble 

 of the struggle for existence the victim and its 

 foe are but rarely well-matched. Take as an 

 example the case of a flycatcher. "This bird," 

 writes D. Dewar, "will sometimes take three 

 or four insects in the course of one flight ; all 

 are captured with the same ease, although the 

 length of wing in each victim varies. So great 

 is the superiority of the bird that it does not 

 notice the difference in the flying powers of its 

 puny quarry." It is unnecessary to labour this 

 point. 



9. Species or varieties differing considerably 

 in colour may exist side by side, as the hooded 

 and carrion crows, the white and dark breasted 

 forms of the Arctic skua, the pale and dark forms 

 of the fulmar petrel, the grey and rufous forms of 

 the American scops owl (Megascops asio). 



It is true that preponderance of one form or 

 another in certain districts points to some advan- 

 tage possessed by one over the other, but, for 

 all we know, it may be due to heredity, and in 

 any case the co-existence of the two types in part 

 of their range, or at certain seasons, shows that 

 selection is not at all rigorous. 



The same argument applies to the co-existence 

 of very differently-coloured species with generally 



44 



